Windows XP SP2 clients "Disassociating" with domain

Hey Guys.

One of the labs at my job is having a huge problem. There are about 10 machines running Windows XP sp2 and they are joined to the Domain on a Windows 2k3 server. Occasionally however they are no longer joined to the domain and have to manually join.I think that the setup for this domain is weird maybe it's just me.

Well there is this program that we use on several computers. The data for that program is stored on the server.

For some reason the guy who runs this lab has the students logon as guest during "normal" operation. and When this program mentioned above is used he has the users logon as the administrator. Why administrator? I have no Idea. So they logon as administrator and attempt to open the program and it runs fine until days later. Eventually the machines can no longer connect to the domain. When attempting to logon the screen says something along the lines of "Domain not available" but I can go to the server and ping the clients fine. It's weird, So every so often he has to manually rejoin the domain on certain machines. I'm totally confused. I don't think he setup the server correctly I mean, should the Windows server or the SonicWall router be the DHCP/DNS server? and I think It may be a problem with the windows firewalls and stuff. So this saturday I want to see what happens if we use static IP's on the machines, however I've seen road runner change the DNS server in the past and I would hate to have to manually reconfigure the DNS settings for all of the machines.

What would make the machines revert back to using the default workgroup?... and would static Ip addresses and modified host files solve the problem of finding the domain controller?

Attatched is a simplified layout of the network.

Just a question:

The Windows XP SP2 machine that students log on to, is it just a standard Windows XP SP2 machine or a part of SOE for that particular lab?